The present invention is directed to a measurement sensor for measuring length or distance changes, particularly for non-contacting measurement of torques of rotating shafts, wherein the length or distance changes to be measured are amplified and converted into an electrical quantity.
Non-contacting, precise measurement of the output torque and, thus, of the mechanical power of rotating shafts is currently one of the most urgent sensing-related problems in energy and automation technology. The fields of application are in monitoring and regulating drives as well as in efficiency optimization of energy conversion systems.
For non-contacting measurement of torques, methods presently known make use either of the acquisition of the mechanical tension generated by the moment of the shaft surface or of the measurement of the torsion.
For converting the mechanical tension into an electrical quantity, wire strain gauges are attached to the shaft and the measured signal is taken via a carrier frequency of the rotating shaft. The method has been known for a long time, but has been predominantly used in the laboratory domain since the application of the wire strain gauges to the shaft is complex and the metrological transmission is very costly. Recent developments are also known wherein the change in the permeability of amorphous metal layers due to mechanical stresses is exploited for torque measurement. Here, however, there are still problems in the application of the amorphous metal layers onto the shaft.
The torsion of a shaft for standard shaft diameters and torques is extremely low. For a 70 mm-thick shaft of an electric motor, for example, the torsion at nominal torque amounts to only a few micrometers at a measuring distance of 30 mm. In order to increase the torsion distance, a longer measuring shaft is joined to the shaft end and the torsion is taken in a non-contacting fashion via inductive systems. This method, however, is not practical since space for the additional measuring shaft is usually not present. Nonetheless, in order to render the torque measurable via torsion of a shaft end that is usually short, the publication "VDI-Nachrichten", No. 20, 15 May 1987, discloses a method wherein the torsion path on the shaft is converted into an axial movement by a mechanical lever system. The axial displacement in the region of tenths of a millimeter is taken in a non-conducting fashion via an inductive system. The axial measuring distance on the shaft thereby corresponds to about twice the shaft diameter. Such an acquisition of torques, however, requires a relatively complex mechanism, however the obtainable precision of about 5% is not adequate for many applications.